I have a 1999 F150 2WD. When I am traveling at highway speeds and start to brake the back end starts to vibrate, shudder, and shimmy a lot, as I slow down it turns into more of a pulsating feeling. I have changed both the front and rear brake and have put new drums on the rear. Whe I changed the drums I noticed that the driver side Lug plate (plate where my lug bolts are mounted) moves inward and outward from the axle a little. After changed the brakes and drums I still have the problem. Could the extra play in the in this area be a bad bearing? Please help, I don't know what to do.

 

That little bit of play is normal in these semi floating style axles. If there is vertical play in the axle shaft then you have a bearing failure. Have you rotated the tires? You may have a tire out of balance. Did you have the front rotors turned or replace when you did the brakes?

 

This sounds more like a driveshaft angularity problem to me. Pinion is too high!

I would have rear spring plates checked for tightness first (they may be loose).

D/S angularity needs to be set from 1 1/2 degress difference (maximum) in operating angle of u-joint A (front) vs. u-joint B (rear).

This can be easily checked with a liquid filled protractor. Vehicle should be on level ground & not jacked-up or elevated.